Traditionally Light Emitting Diodes (LEDs) have primarily been used as indicator lamps in electronic equipment. However recently the power and efficacy (e.g., lumens per watt of electrical power) has been increasing and LEDs have been identified as a possible replacement for inefficient incandescent bulbs.
Like incandescent light bulbs, LEDs generate a certain amount of heat. Whereas incandescent light bulbs radiate a large amount of heat as infrared radiation in the case of LEDs heat must mainly be dissipated by conduction and convection. It is important to prevent the temperature of LEDs from rising too much because as the temperature of the LED increases its light output, efficacy and lifetime decrease. Thus, the promised benefits of LED lighting in terms of efficacy and lifetime can be diminished. A single power LED produces several tens of lumens, so in order to match the light output of an incandescent light bulb ten or more LEDs would typically be required. More LEDs mean a greater heat load.
FIG. 1 shows a prior art LED light bulb 100. A set of four rectangular circuit boards 102 (two of which are visible) are arranged in a box formation. A smaller square circuit board 104 is arranged to close a top end of the box formation. LEDs 106 are mounted on the circuit boards 102, 104. The arrangement of circuit boards 102, 104 is enclosed in a space formed between a bulb shaped cover 108 and a lower plastic housing 110.
From a thermal standpoint, this arrangement tends to trap heat within the enclosed space leading to a relatively limited power handling capacity of such bulbs. At such low powers as can be used with this design, the light produced is inadequate to match a conventional incandescent lamp or a compact fluorescent lamp.
From an optical point of view, the light distribution is best described as ad hoc. The LEDs used may be Lambertian emitters or more directional than that so that the LEDs on the rectangular circuit boards 102 provide illumination that is highly peaked along the direction perpendicular to the boards 102 surfaces and likewise the LEDs on the smaller square top board 104 provide illumination that is highly peak in the upward direction. Generally there is no expectation or realization of illumination patterns that blend together to provide anything approaching uniform illumination. Thus, optically this bulb inadequately matches the light distribution of standard incandescent bulbs.